In recent years, environmental issues such as global warming believed to be caused by an increase in CO2 have become serious. Research and development of eco-friendly solar cells utilizing sunlight as a clean energy source have been actively conducted. Among such solar cells, dye-sensitized solar cells have been gaining attention as low-cost solar cells offering improved photoelectric conversion efficiency.
A dye-sensitized solar cell is formed by laminating, for example, a transparent substrate, a transparent conductive layer formed on the transparent substrate, an oxide semiconductor layer carrying a dye, an electrolyte layer containing a redox pair and an electrolyte, and a substrate on which a counter electrode has been formed, in such order from the light incidence side. In particular, Grätzel cells are characterized by a porous oxide semiconductor layer obtained by calcinating nanofine particles of titanium oxide. The use of a porous oxide semiconductor layer results in an increase in the amount of a sensitized dye to be adsorbed, thereby improving photoabsorption performance.
In a method for producing the above dye-sensitized solar cell, for example, a porous semiconductor layer comprising titanium oxide particles is formed in advance on a transparent conductive layer formed on the surface of a transparent substrate and a dye is carried on the porous semiconductor layer. Next, a counter electrode is coated with a catalyst made of a platinum film or the like. The semiconductor layer and the platinum film are layered such that they face to each other. An electrolyte is injected into the space therebetween to form a electrolyte layer. The sides of the space are sealed with an epoxy resin or the like. Thus, a dye-sensitized solar cell is produced.
However, liquid electrolytes have been conventionally used for the electrolyte layer. Therefore, there is a risk of liquid leakage due to deterioration or destruction of a sealing material. This causes reduction of photoelectric conversion efficiency, which is problematic. In order to solve such problem, many types of dye-sensitized solar cells each comprising an electrolyte layer that has been solidified using a high-molecular compound to prevent liquid leakage have been suggested.
For example, Patent Literature 1 discloses a solar cell comprising a photoelectrode, a counter electrode, and an electrolyte provided between the photoelectrode and the counter electrode, in which the electrolyte contains a high-molecular compound having a radius of inertia of 100 Å to 1000 Å. According to this invention, a low-crystalline compound such as polyethylene oxide or polyethylene glycol is used as a high-molecular compound. Such compound has a low-melting point. This results in insufficient shell durability, which has been problematic.
In addition, Patent Literature 2 discloses a dye-sensitized solar cell having a structure in which a dye-sensitized semiconductor electrode is formed by allowing a dye to be adsorbed by a porous film of an oxide semiconductor formed on a substrate and an organic medium in which an electrolyte has been dissolved is allowed to come into contact with the electrode, and the organic medium containing an electrolyte dissolved therein is solidified using a natural polymer such as cellulose or a derivative thereof. Cellulose does not negatively affect cell performance and has high thermostability. Therefore, it is preferable to use cellulose as a high-molecular compound for an electrolyte. However, the addition of cellulose inhibits ion conductivity. As a result, conversion efficiency tends to decrease.
Further, Patent Literature 3 discloses a solid electrolyte used for a dye-sensitized solar cell and the like, in which an electrolyte is carried by a three-dimensional crosslinked construct formed by allowing a compound containing a reactive functional group such as a cellulose having a hydroxyl group to react with a compound containing an isocyanate group capable of reacting with the functional group. However, an electrolyte solidified via such crosslinking reaction has no ion conductivity. Therefore, it is thought that such electrolyte cannot actually function as an electrolyte layer in a dye-sensitized solar cell.
Meanwhile, ionic liquid (molten salt) is added to prevent reduction of conversion efficiency caused by a high-molecular compound (Patent Literature 4). However, it is necessary to use a large amount of ionic liquid in order to achieve sufficient conversion efficiency. In this case, it is also necessary to increase the amount of the high-molecular compound added to retain ionic liquid. This eventually results in reduction of conversion efficiency. Thus, a vicious cycle is created.
In addition, lithium iodide has been conventionally used as a substance that constitutes a redox pair contained in an electrolyte layer (Patent Literature 5). Lithium iodide has high deliquescent properties and thus tends to deteriorate. Therefore, temporal stability of the electrolyte decreases, resulting in a remarkable decrease in conversion efficiency from the initial level. In addition, lithium iodide itself can spoil over time. Therefore, it has been difficult to handle lithium iodide. In addition, the initially obtained conversion efficiency and durability have been insufficient.